Sense amplifier circuit

ABSTRACT

An equalization circuit may include a first sense amplifier having an input, the input being electrically isolated from an input to a second sense amplifier. An equalizer may be connected to the input to the first sense amplifier to provide an equalizing voltage to the input to the first sense amplifier. The input to the first sense amplifier may be equalized by the equalizing voltage independent from the input to the second sense amplifier.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This present invention relates generally to the field of integratedcircuit design and, more specifically, to an improved sense amplifiercircuit design.

2. Description of the Related Art

This section is intended to introduce the reader to various aspects ofart that may be related to various aspects of the present invention,which are described and/or claimed below. This discussion is believed tobe helpful in providing the reader with background information tofacilitate a better understanding of the various aspects of the presentinvention. Accordingly, it should be understood that these statementsare to be read in this light, and not as admissions of prior art.

Memory devices, such as static random access memory (“SRAM”) and dynamicrandom access memory (“DRAM”), may include arrays of memory cells thateach store a bit or element of data. Each data bit or element mayrepresent a logical low (“0”) or a logical high (“1”), which maycorrespond to a voltage level of a circuit element disposed within thememory cell. As an example, a voltage level close to ground may berepresentative a logical low or “0” and a voltage level close to asupply voltage for the memory device may be representative of a logicalhigh or “1.” The electrical paths that carry the voltage representing abit or element of data so that it may be accessed may be referred to asa bitline.

Bitlines may be precharged before the data stored in associated memorycells is accessed. Precharging the bitline may allow faster access tothe data stored in the memory cells. In precharging, the voltage levelof a bitline is equalized to a voltage in between the voltage levelsthat correspond to logical lows and logical highs. Thus, when thebitline is accessed and begins to change voltage level to the voltagelevel corresponding to the data value stored therein, the voltage valueon the bitline will only have to transition about half of the rangebetween a logical low and a logical high.

Sense amplifier circuits are coupled to the bitlines to access datastored in a memory cell. A sense amplifier amplifies a signalcorresponding to the difference between the voltage level of a bitlineand the voltage level to which the bitline is being driven to representa data value. When bitlines are equalized during precharging, however,the bias voltage presented to a sense amplifier may inhibit theoperation of the sense amplifier, making output performance of the senseamplifier slower. Another potential problem is that some bitlines may betoo long to be effectively equalized in the time available betweenmemory access cycles. These long lines are typically on the order of10,000 micrometers (0.01 meters) and are typically global data lines ina memory. This may be true because of the inherent resistor-capacitor(“RC”) delay associated with the long lines.

BRIEF DESCRIPTION OF THE DRAWINGS

Advantages of the invention may become apparent upon reading thefollowing detailed description and upon reference to the drawings inwhich:

FIG. 1 is a block diagram of an electronic device containing integratedcircuit devices that may employ embodiments of the present invention;

FIG. 2 illustrates a portion of a memory array that may employembodiments of the present invention;

FIG. 3 illustrates an electric schematic diagram of an exemplary memorycell in the array of FIG. 2;

FIG. 4 is a block diagram showing a conventional sense amplifiercircuit;

FIG. 5 is a block diagram showing a sense amplifier circuit according toan embodiment of the present invention; and

FIG. 6 is a process flow diagram showing the operation of a senseamplifier circuit in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

One or more specific embodiments of the present invention will bedescribed below. In an effort to provide a concise description of theseembodiments, not all features of an actual implementation are describedin the specification. It should be appreciated that in the developmentof any such actual implementation, as in any engineering or designproject, numerous implementation-specific decisions must be made toachieve the developers' specific goals, such as compliance withsystem-related and business-related constraints, which may vary from oneimplementation to another. Moreover, it should be appreciated that sucha development effort might be complex and time consuming, but wouldnevertheless be a routine undertaking of design, fabrication, andmanufacture for those of ordinary skill having the benefit of thisdisclosure.

FIG. 1 is a block diagram of an electronic device containing integratedcircuit devices that may employ embodiments of the present invention.The electronic device or system, which is generally referred to by thereference numeral 10, may be any of a variety of types such as acomputer, pager, cellular phone, personal organizer or the like. In aprocessor-based device, a processor 12, such as a microprocessor, maycontrol the operation of system functions and requests. The processor 12may be coupled to various types of memory devices to facilitate itsoperation. For example the processor 12 may be connected to a volatilememory 26 and a non-volatile memory 28. The volatile memory 26 maycomprise a variety of memory types, such as static random access memory(“SRAM”) or dynamic random access memory (“DRAM”) or the like. Thenon-volatile memory 28 may comprise various types of memory such aselectrically programmable read only memory (“EPROM”), and/or flashmemory or the like.

The system 10 may include a power supply 14, which may comprise abattery or batteries, an AC power adapter and/or a DC power adapter.Various other devices may be coupled to the processor 12 depending onthe functions that the system 10 performs. For example, an input device16 may be coupled to the processor 12 to receive input from a user. Theinput device 16 may comprise a user interface and may include buttons,switches, a keyboard, a light pen, a mouse, a digitizer and/or a voicerecognition system or the like. An audio or video display 18 may also becoupled to the processor 12 to provide information to the user.

A communications port 22 may be adapted to provide a communicationinterface between the electronic device 10 and peripheral devices 24.The peripheral 24 may include a docking station, expansion bay or otherexternal component. Furthermore, an RF sub-system/baseband processor 20may be coupled to the processor 12 to provide wireless communicationcapability.

The processor 12, the non-volatile memory 28 and the volatile memory 26may be implemented as one or more integrated circuit components. Also,the processor 12, the non-volatile memory 28, and the volatile memory 26are examples of integrated circuit components that may include senseamplifier circuits constructed in accordance with embodiments of thepresent invention.

FIG. 2 illustrates a portion of a memory array, which may be included inone or more of the integrated circuit devices illustrated as part of theelectronic device 10 in FIG. 1. The memory array is referred togenerally by the reference numeral 100. As can be seen, the memory array100 includes a plurality of memory cells 102 that are arranged ingenerally perpendicular rows and columns. The memory cells 102 in eachrow are coupled together by a respective word line 104, and the memorycells 102 in each column are coupled together by a respective bitline ordigit line 106. Specifically, each memory cell 102 includes a word linenode 108 that is coupled to a respective word line 104, and each memorycell 102 includes a digit line node 110 that is coupled to a respectivebitline 106. The conductive word lines 104 and bitlines 106 arecollectively referred to as address lines. These address lines may beelectrically coupled to an integrated circuit such as a processor ormemory controller so that each of the memory cells 102 can be accessedfor storage and retrieval of information.

FIG. 3 illustrates an exemplary memory cell 102 that may be used in thememory array 100. The memory cell 102 includes a memory element 112 thatmay be coupled to an access device 114. The memory element 112 may be acapacitive memory element such as a storage capacitor, and the accessdevice 114 may be a MOSFET transistor. Capacitors may be used as thestorage element in DRAM or other memory types. The base of thetransistor 114 may be coupled to the word line 104 to form the word linenode 108, and the source of the transistor 114 may be coupled to thebitline 106 to form the bitline node 110. One plate 116 of the memoryelement 112 may be coupled to the drain of the transistor 114, and theother plate 118 of the memory element 112 may be coupled to a voltagelevel V_(cc), which is typically circuit ground.

FIG. 4 is a block diagram showing a conventional sense amplifiercircuit, which is generally referred to by the reference numeral 200. Afirst sense amplifier 202 and a second sense amplifier 210 are connectedto each other by a bitline 212 and a bitline 214. The bitlines 212 and214 may correspond to two of the bitlines 106 illustrated in FIG. 2. Anequalizer 204, an equalizer 206, and an equalizer 208 are connected inparallel between the sense amplifier 202 and the sense amplifier 210.

The equalizers 204, 206, and 208 may be used to precharge the bitlines212 and 214 prior to reading data from associated memory cells such asthe memory cells 102 (FIG. 3). For purposes of example, the bitlines 212and 214 may be long lines that require multiple equalizers for effectiveequalization at desired data access rates. The inherent RC delay in thelayout of the bitlines 212 and 214 may contribute to the need formultiple equalizers to effectively precharge the inputs to the senseamplifiers 212 and 214. Even with multiple equalizers, it may bedifficult to effectively equalize the bitlines 212 and 214 at highaccess rates. The inherent RC delay caused by the length of the bitlinesmay mean that effective equalization cannot occur given the relativelyshort equalization times that are available at high access frequencies(greater than about 200 Megahertz (MHz) core frequency).

FIG. 5 is a block diagram showing a sense amplifier circuit according toan embodiment of the present invention. Those of ordinary skill in theart will appreciate that, while the exemplary embodiment shown in FIG. 5involves the equalization of bitlines in a memory device, the presentinvention may have applicability in any environment in which it isdesirable to equalize long lines having an inherently high RC delayrelative to desired performance frequencies. Those of ordinary skill inthe art will appreciate the applicability of the invention to theequalization of other types of data lines, such as global data lines, inaddition to bitlines. The sense amplifier circuit is generally referredto by the reference numeral 300. A first sense amplifier 302 isconnected to a long bitline 310 and a long bitline 316. The long bitline310 is capacitively coupled by a capacitor 312 to a short bitline 314,which is connected to a second sense amplifier 308. Short bitlines aretypically subject to parasitic loading (resistance and capacitance),which is very small. The long bitline 316 is capacitively coupled by acapacitor 318 to a short bitline 320, which is connected to the secondsense amplifier 308. The use of the capacitive coupling provided by thecapacitors 312 and 318 may improve the performance of the second senseamplifier by decoupling the short bitlines 314 and 320 from the longbitlines 310 and 316. This decoupling may allow a reduction in theoverall number of equalizers required for a sense amplifier circuit.

For purposes of example, the sense amplifier circuit 200 (FIG. 4)without capacitive coupling includes three equalizers 204, 206 and 208,while the sense amplifier circuit 300 includes only two equalizers 304and 306. An equalizer 304 is connected in parallel between the longbitlines 310 and 316, and an equalizer 306 is connected in parallelbetween the short bitlines 314 and 320. The reduction in number ofequalizers is possible because the RC delay inherent in the shortbitlines 314 and 320, as well as the RC delay inherent in the longbitlines 310 and 316, is reduced. The reduction in RC delay means thatfewer equalizers are needed to equalize all the bitlines 310, 314, 316and 320 because equalization can occur in a sufficiently short timeperiod relative to the operating frequency of the associated memorydevice. As an example, embodiments of the invention may allowequalization can take place without affecting operational parameters ofthe memory device such as read-after-write time, which is the timeperiod that valid data is available from a memory cell after beingwritten to the memory cell.

The capacitors 314 and 318 isolate the short bitlines 314 and 320 fromDC voltage levels provided by the equalizer 304 to precharge the longbitlines 310 and 316. This isolation makes biasing the short bitlines314 and 320 with a precharge voltage easier because precharging theshort bitlines 314 and 320 may be done without regard to biasingvoltages present on the long bitlines 310 and 316. Thus, maintaining theprecharge level of the short bitlines 314 and 320 at optimum levels maybe made easier. Because of the capacitive coupling provided by thecapacitors 312 and 318, only high frequency signal components (on theorder of about 318 MHz for a coupling capacitance of 0.5 picofarads anda long line resistance of 1,000 Ohms) are transferred between the longbitlines 310 and 316 and the short bitlines 314 and 320, which mayalready be equalized for a given read operation. This means that theperformance of high frequency operations of the second sense amplifier308 may be improved even though the long bitlines 310 and 316 may not befully equalized.

FIG. 6 is a process flow diagram showing the operation of a senseamplifier circuit in accordance with an embodiment of the presentinvention. The process is generally referred to by the reference numeral400. At block 402, the process begins. At block 404, data is stored in amemory cell associated with a sense amplifier circuit. The senseamplifier circuit may correspond to the sense amplifier circuit 300(FIG. 5). At block 406, a short bitline, which may correspond to one ofthe short bitlines 314 and 320 of FIG. 5, that is connected to the senseamplifier may be equalized in isolation from a long bitline, such as oneof the long bitlines 310 and 316 of FIG. 5. The isolation of the shortbitline may be provided by a capacitor, such as one of the capacitors312 and 318. At block 408, data is read from the sense amplifier atblock 408. As set forth above, the isolation of the short bitline mayimprove high frequency performance of the sense amplifier circuit byallowing the short bitline to be equalized more quickly. Additionally,the equalization of the short bitline may be made easier because theisolation of the short bitline from the long bitline may be performedindependently of any DC equalization voltage present on the longbitline. At block 410, the process ends.

While the invention may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and will be described in detail herein. However,it should be understood that the invention is not intended to be limitedto the particular forms disclosed. Rather, the invention is to cover allmodifications, equivalents and alternatives falling within the spiritand scope of the invention as defined by the following appended claims.

1. A method of accessing a memory device, comprising: storing data in amemory cell associated with a sense amplifier circuit; equalizing afirst bitline segment associated with both the sense amplifier and asecond bitline segment, wherein equalizing the first bitline segmentcomprises equalizing the first bitline segment isolated from the secondbitline segment by a capacitor; transmitting a signal indicative of datastored by the memory cell from the second bitline segment to the senseamplifier through the first bitline segment; and reading the data storedby the memory cell via the sense amplifier circuit.
 2. The method setforth in claim 1, wherein the recited acts are performed in the recitedorder.
 3. The method set forth in claim 1, wherein equalizing the firstbitline segment comprises equalizing the first bitline segmentassociated with the sense amplifier in isolation from the second bitlinesegment associated with the sense amplifier.
 4. The method set forth inclaim 1, wherein the first bitline segment is substantially shorter thanthe second bitline segment.
 5. The method set forth in claim 1, whereinreading data comprises amplifying a signal corresponding to a differencebetween an equalized voltage level of the first bitline segment and avoltage level to which the bitline is being driven to represent a datavalue.
 6. The method set forth in claim 1, comprising equalizing thefirst bitline segment in isolation from the second bitline segment bothbefore and after reading data from the memory cell.
 7. A method ofoperating a memory device, comprising: isolating a sense amplifier froma first bitline signal having a first frequency; and transmitting asecond bitline signal having a second frequency to the sense amplifier,wherein the second frequency is substantially higher than the firstfrequency.
 8. The method set forth in claim 7, wherein the first bitlinesignal is a direct current signal from an equalizer and the firstfrequency is approximately zero.
 9. The method set forth in claim 7,wherein the second frequency is greater than 318 MHz.
 10. The method setforth in claim 7, wherein the second bitline signal is from a capacitivememory element.
 11. The method set forth in claim 7, whereintransmitting comprises transmitting between plates of a capacitor bycapacitively coupling a first bitline segment connected to a memoryelement and a second bitline segment connected to the sense amplifier.12. The method set forth in claim 7, comprising independentlyprecharging a first bitline segment coupled to a memory element and asecond bitline segment coupled to the sense amplifier.
 13. The methodset forth in claim 7, comprising receiving a data value corresponding tothe second bitline signal in a processor.
 14. A method, comprising:charging a first bitline segment; charging a second bitline segment; andtransmitting a signal indicative of data from the first bitline segmentto the second bitline segment through a high-pass filter.
 15. The methodof claim 14, wherein transmitting a signal indicative of data from thefirst bitline segment to the second bitline segment through a high-passfilter comprises transmitting the signal indicative of data through acapacitor coupled to both the first bitline segment and the secondbitline segment.
 16. The method of claim 14, comprising preventingcurrent from flowing between the first bitline segment and the secondbitline segment while charging the first bitline segment.
 17. The methodof claim 14, comprising switchably connecting the second bitline segmentto a data source.
 18. The method of claim 17, wherein the data source isa memory device.